Polyamide polyolefin mixtures are known to combine the advantages of polyolefins (ease of injection molding, chemical resistance, insensitivity to moisture) and of polyamides (good mechanical properties).
Mixtures based on polyamide and polyolefin have been described in Patent Application publication EP 342066. Since polyamides and polyolefins are not very compatible, it is necessary either to graft a proportion of the polyolefin, for example with maleic anhydride (forming a maleinized polyolefin), or to add a third substance (compatibilizing agent). EP 342066 shows the advantage of some third substances for obtaining mixtures of polyamides and polyolefin which have good mechanical properties. However, if these mixtures of polyamide and polyolefin are employed for making articles which have a weld line, ruptures may occur along this line.
The weld line in an article is the meeting of two flows of molten polymers when the article is manufactured, for example by injection molding at two or more points in the mold. At the junction of two or more molten polymer flow fronts, characteristic regions are formed, called weld lines or knit lines. Injection molding of complex articles is practically impossible to carry out without weld lines.
The morphology and the mechanical properties of articles with weld lines differ substantially from those that are devoid of them. In general, the weld lines exhibit an orientation of the material which is parallel to the plane of welding, and give rise to a considerable embrittlement of the articles. In the case of mixtures of incompatible polymers the weld lines have particularly dramatic effects because of the heterogeneous nature of these materials. Increasing the temperatures of the mold and of the material does not suffice to improve appreciably the toughness of the weld lines.
An article by B. Fisa et al., "Effect of an Ionomer Compatibilizer on the Structure and Mechanical Properties of Injection molded PA6/HDPE Blends", which appeared in ANTEC '91, pages 1135-1139, describes the effect of a compatibilizing agent for improving the tensile strength of the weld lines. This agent is an ethylene/methacrylic acid/isobutyl acrylate copolymer. The methacrylic acid is 70% neutralized with zinc.
Patent Application JP 1 284785 of Nov. 2, 1989 published on Jun. 21, 1991 under no. JP 3146552 describes mixtures including 80 to 40% of polyamide, 1 to 40% of a modified polyolefin, and 20 to 60% of polypropylene and such that the ratio of the viscosity of the polypropylene to the viscosity of the polyamide is higher than 0.75 when these viscosities are measured at the molding temperature and at a shear rate of 3500 s.sup.-1. These mixtures are particularly suitable for being painted or metallized. Nothing is mentioned concerning the behaviour of weld lines.